This invention relates to communications apparatus and, more particularly, to apparatus for mitigating signal contrast and signal distortion in a communications system.
In a communications system such as the improved time assignment speech interpolation (TASI) system known as a digital speech interpolation (DSI) system and disclosed in U.S. Pat. No. 4,002,841 issued to Y. Ching-A. M. Hofmann-D. G. Messerschmitt, bandwidth can be reduced by utilizing speech inactivity time. For example, a speech signal from each of N calling parties may be provided over a respective trunk to a transmitter. Situated at the transmitter is a speech detector for detecting the activity status on the trunk, i.e. speech activity and speech inactivity times, as well as for defining time slots on a transmission link between the transmitter and a receiver during which a connection should be made between the calling and a called party. The transmission link includes a plurality of C channels, C being less than N. A channel is selected for the connection responsive to the detection of speech activity. Once selected, the trunk speech signal is extended from the trunk through the transmitter thence over the channel to the receiver. At the receiver, an inverse operation usually occurs. That is, the signal on one of the C channels is supplied to a respective one of a plurality of receiver trunks, and, eventually, to the called party. Later, upon detection of speech inactivity, the connection thus established is released and the channel capacity made available to another active trunk. Thereby, bandwidth between transmitter and receiver is reduced by concentrating signals between a plurality of calling trunks and a plurality of called trunks over a lesser plurality of channels.
In speech interpolation systems of the type disclosed in U.S. Pat. No. 2,541,932, it is common for the speech signal to pass through the transmitter to the receiver, and therethrough to a second transmitter, therethrough to a second receiver, etc., i.e., through a plurality of communications apparatuses which are connected for tandem operation. Unfortunately, the speech detector of a transmitter tends to clip, and hence distort, at least the initial segment of the speech signal. Too, in a tandem system wherein the speech signal passes through a plurality of transmitters, the distortion may tend to accumulate.
Accordingly, an object of the present invention is to mitigate the accumulation of signal distortion in a communications system.
Inasmuch as the calling trunk is not usually connected on a continuous basis to the called trunk, e.g., during periods of speech inactivity, the called party may perceive the disconnection with the calling party. Thus, during speech inactivity, it is well known for receiver communications apparatus to insert a noise signal over the called trunk so that the called party perceives the continued existence of the connection when there is no connection. Notwithstanding, the called party may object to an annoying contrast between the inserted noise and the noise commonly present on the calling trunk, for example, Gaussian noise.
Therefore, another object of the present invention is to mitigate noise signal contrast in a communications system.